W114 250C Blowing #4 fuse

[marken]

I recently purchased a 1972 250C. It will run fine for a somewhat short time but then Fuse #4 blows. Once blown the car won’t idle and stalls when put in gear or reverse. Prior to when the fuse blows the right turn signal works but not the left. After blowing neither turn signal works. Any suggestions on tracking down what may be causing the fuse to blow? Also, the emergency flashers don’t work. As a side note there was a white 8 amp fuse in the space when I got the car. I was told by the previous owner to put in a red 16 amp fuse and give that a try.

I am not too mechanically inclined so possibly I shouldn’t have vintage cars but I really do like them.

Kevin Parks
1972 250C
1985 280TE

[Popeye]

I'm really not sure... but a few thoughts:

- What is the exact sequence of events prior to fuse blowing? Start car with lights off, let it idle, then fuse blows? Or drive the car? Are the lights being turned on (turn signals?) after starting - in turn causing the fuse to blow?

- "Automatic starting help" sounds like the electric choke. (Smarter folks than me, please chime in.) Can you start the car with the fuse blown, and give it gas to keep it running until it warms up? If so, does it idle on its own?

- turn signals... you might have to chase wires to look for missing insulation, that could be shorting - perhaps intermittently. I would start inside the signal housings themselves; this is where the wires are most exposed to the elements (leaks in the housing, heat of the bulb, wire connections, etc.).

- Any chance the bulbs are oversized? Perhaps an exercise would be to remove all the bulbs. If that prevents the fuse from blowing, replace bulbs one at a time?

None of this is fast, and assumes you have a large supply of fuses at hand. Again, perhaps smarter / more experienced folks than me will chime in with better ideas?

Good luck - let us know what you find! These are lovely cars!

[Idle]

I once had this problem on an Opel. The fault was a turn signal bulb socket has rusted and the power was shorting out from all the rust built up around the bulb.

So I took all the turn signal bulbs out, cleaned their sockets, and after that everything worked.

This might not be where your short is but if you have the time this is something every fifty year old car needs eventually.

[cth350]

What it refers to as a "starting aid" is the wiring to your carbs. There should be leads going to the choke covers. But also, if your carbs have fuel cutoff solenoids, there are wires to them too.

The fact that your car isn't starting suggests that you have fuel cutoff solenoids.

So that leaves you with four possible places for the failure....
1. The choke covers
2. The fuel cut-offs
3. The tail lights
4. The wiring that connects all of the above, including the turn indicator switch & flasher.

I would consider doing the following...

A. See if you can ix the turn signal that never works. This is an unknown that messes up diagnosing the rest.
B. Remove the working tail(s) light and see if the problem goes away.
C. Disconnect the leads to the choke covers. Tape them to keep them from shorting to anything. See if the problem goes away. The car should start in relatively warm weather without them. It may not run so well with them disconnected after it gets warm; humor it.
D. Disconnect the leads to the fuel cut-off solenoids and tape them. Create a replacement for them that has an inline fuse and connect it where the radio fuse is (so that both the inline fuse AND the radio fuse are in the circuit). Report what happens.


To build your own fuel cut-off lead, eyeball it. The original should be a single female socket to each carb. But a 5-pack of similar sockets. Any reasonable autoparts store should have them. Connect them together with stranded 14 guage wire to an inline fuse. To figure out the correct end of the radio fuse, remove the fuse from the little fuse box it's in and see which of the two leads still has 12 volts (when the key is turned).

A warning about multimeters. The ohms & continuity setting is only good for dead circuits. Best to disconnect the car battery while trying to figure out continuity with a meter. If you are real careful, you don't need to do. If you're really, really careful, you'll disconnect the battery.

For that dead tail light. You've be using your meter on the DC Voltage setting at first, checking to see if the light bulb has 12v to ground one 1x lead. If neither end of the bulb has 12v work your way back forward looking for where the last place 12v can be found. If both ends of the bulb have 12v, then you are looking for a bad ground between the bulb and the body. (Like the rusty opel observation already made). And of course, if you do indeed find that only 1x lead has 12v, you have a bad lightbulb.

Just remember while trying to diagnose stuff, fix one thing at a time and then re-test. After all, you might have more than 1x thing broken.

-CTH